In recent years, attention has been given to a display in which a contact detection device (hereinafter referred to as “touch sensor”) or a so-called touch panel is directly mounted on a liquid crystal display and various buttons are displayed on the liquid crystal display so that information is allowed to be inputted by the buttons instead of normal buttons. This technique provides significant advantages of saving space and reducing in the number of components when there is a trend toward larger screen sizes in mobile devices, because a display and buttons are allowed to be located in a common area. However, mounting the touch sensor on the liquid crystal display causes an increase in the whole thickness of a liquid crystal module. In particular, in the case where the touch sensor is applied to a mobile device, a protective layer for preventing scratches on the touch sensor is necessary, so liquid crystal modules have a tendency to have a larger thickness, and this tendency goes against a trend toward thinner liquid crystal modules.
Therefore, for example, in Japanese Unexamined Patent Application Publication No. 2008-9750 (hereinafter “PTL 1”), a liquid crystal display element with a touch sensor in which a capacitive type touch sensor is formed, is proposed so as to reduce the thickness of the liquid crystal display element. In this case, a conductive film for the touch sensor is arranged between an observation-side substrate of the liquid crystal display element and an observation-side polarizing plate arranged on an outer surface of the observation-side substrate, and the capacitive type touch sensor which has an outer surface of the polarizing plate as a touch surface is formed between the conductive film for the touch sensor and the outer surface of the polarizing plate.
However, in the liquid crystal display element with the touch sensor disclosed in the above-described PTL 1, in principle, it is necessary for the conductive film for the touch sensor to have the same potential as that of a user, and it is necessary for the user to be properly grounded. Therefore, aside from stationary televisions drawing power from an outlet, it is realistically difficult to apply the liquid crystal display element with the touch sensor to mobile devices. Moreover, in the above-described technique, it is necessary for the conductive film for the touch sensor to come very close to a finger of the user, so a position where the conductive film for the touch sensor is arranged is limited, for example, the conductive film for the touch sensor is not allowed to be arranged deep in the liquid crystal display element. That is, the degree of flexibility in design is low. Moreover, in the above-described technique, because of the configuration, it is necessary to arrange a circuit section including a touch sensor drive section or a coordinate detection section separately from a display drive circuit section of the liquid crystal display element, so integration of circuits for a whole apparatus is difficult.
Therefore, in addition to a touch panel directly mounted on a liquid crystal display, for an example, it is considered that a touch detection electrode forming a capacitance between the touch detection electrode and the common electrode is arranged in addition to a common electrode originally arranged for application of a display common voltage. Because the capacitance changes depending on whether or not an object touches or comes close to the touch detection electrode, the display common voltage to be applied to the common electrode by a display control circuit is also used (doubles) as a touch sensor drive signal. Accordingly, a detection signal in response to a change in capacitance is obtained from the touch detection electrode. Then, when the detection signal is inputted into a predetermined touch detection circuit, whether or not the object touches or comes close to the touch detection electrode is detectable. Moreover, by this technique, a display with a touch sensor allowed to be applied to a mobile device of which a user often has an unsteady potential is obtainable. Moreover, a display with a touch sensor having a high degree of flexibility in design according to the type of a display layer is obtainable, and a circuit for display and a circuit for sensor are easily integrated into one circuit substrate, so there is an advantage of easy circuit integration.
Moreover, in various capacitive type touch sensors, to prevent a touch detection electrode for obtaining a detection signal in response to a change in capacitance from being floating, it is considered that the touch detection electrode is grounded through a resistor (e.g., a grounding resistor).
However, in such a configuration, a sensor output is leaked through the grounding resistor (e.g., a leakage current flows from a signal line for the detection signal to the ground). Therefore, it is considered that an appropriate detection signal may not be obtained due to a decrease in the range of a detection signal value or a decrease in a signal value with time.